Actively-cooled plasma electrode for long pulse operations in a cesium-seeded negative ion source

Fujiwara, Yukio; Watanabe, Kazuhiro; Okumura, Yoshikazu; Trainham, R.*; Jacquot, C.*

A new actively-cooled plasma electrode has been developed for long pulse operation in a cesium-seeded negative ion source. To keep the electrode temperature at about 300 C, which is optimum range of temperature to enhance cesium effects, the electrode cooling structure has been designed using three-dimensional numerical simulation assuming that the heat flux from the source plasma was 15 W/cm. Water cooling tubes were brazed to the plasma electrode substrate with spacers made of stainless steel which acts as a thermal resistance. The fabricated plasma electrode has been tested in a cesium-seeded volume negative ion source called Kamaboko source. The temperature of the electrode reached 280 C for the arc power of 41 kW, which is operating condition required for producing D- beams with current densities exceeding 20 mA/cm. It was demonstrated that the actively-cooled plasma electrode is applicable to long pulse operations, meeting the temperature requirement for optimizing the surface-production process of negative ions in the cesium seeded ion source.